The biosphere is inherently chiral; each class of biological macromolecules is rade up of monomer molecules of uniform chirality (Mason, Chirality 3:223, 1991) and the biochemical interactions of biological macromolecules are inherently chiral.
Enzymes, for example, invariably act only on one enantiomer of a chiral substrate, or generate only one diastereomer from a prochiral substrate. Fersht, in “Enzyme Structure and Mechanism”, W.H. Freeman and Company, San Francisco, 1977, pp. 75–81. This specificity can be related to the chiral structure of the enzyme molecule, including the three-dimensional folding of the polypeptide backbone and the orientation of the amino acid side chains in the folded protein molecule. Fersht, supra. To date only L-enzymes have been described in nature; this leaves the description of D-enzymes and their properties, which include folded structure, enzymatic activity, and chiral specificity, as unexplored questions.
Recently, Zawadzke et al., J. Am. Chem. Soc., 114:4002–4003, 1992, described the preparation of a small 45 amino acid residue polypeptide (D-rubrodoxin) using D-amino acids. L-rubrodoxin is found in clostridia and is the simplest iron-sulfur protein. It is believed to function in electron transport. However, it lacks an demonstrated enzymic activity.
Prior to the present invention, the largest L-protein known to be chemically synthesized in a conventional step-wise fashion is Preprogonadotropin Release Hormone (PreproGnRH). PreproGnRH has 93 amino acid residues. (Milton et al., Biochemistry, (1992) 31: 8800.) PreproGnRH inhibits prolactin release.
Many organic compounds exist in optically active forms, i.e., they have the ability to rotate the plane of plane-polarized light. In describing an optically active compound, the prefixes D and L or R and S are used to denote the absolute configuration of the molecule about its chiral center(s). The prefixes (+) and (−) or d and l are employed to designate the sign of rotation of plane-polarized light by the compound, with (−) or l meaning that the compound is levorotatory. A compound prefixed with (+) or d is dextrorotatory. For a given chemical structure, these compounds, called stereoisomers, are identical except that they are mirror images of one another. A specific stereoisomer may also be referred to as an enantiomer, and a mixture of such isomers is often called an enantiomeric or racemic mixture.
The property of optical activity is due to molecular asymmetry about carbon atoms that are linked to four different atoms or molecules. Where there is only one asymmetric carbon atom, or chiral center as it is sometimes called, there are two possible stereoisomers. Where there are n asymmetric carbons or chiral centers, the number of potential stereoisomers increases to 2n. Thus, a molecule with three chiral centers would have eight possible stereoisomers.
While the structural differences between stereoisomers are subtle and of little consequence in ordinary chemical reactions, they may be profound where biological systems are concerned, i.e., if the compounds are utilized in enzyme-catalyzed reactions. Thus, the L-amino acids are readily metabolized in humans but the corresponding D-analogs are not, and only D-glucose can be phosphorylated and processed into glycogen or degraded by the glycolytic and oxidative pathways of intermediary metabolism. Similarly, beta blockers, pheromones, prostaglandins, steroids, flavoring and fragrance agents, pharmaceuticals, pesticides, herbicides, and many other compounds exhibit critical stereospecificity. In the field of pesticides, Tessier [Chemistry and Industry, Mar. 19, 1984, p. 199] has shown that only two of the eight stereoisomers of deltamethrin, a pyrethroid insecticide, have any biological activity. The same statement concerning the concentration of bioactivity in a single isomer can be made about many other pesticides, including the phenoxypropionates and halopropionate derivatives, each containing one chiral center and existing in the form of two optical isomers.
Stereochemical purity is of equal importance in the field of pharmaceuticals, where 12 of the 20 most prescribed drugs exhibit chirality. A case in point is provided by naproxen, or (+)-S-2-(6-methoxy-2-naphthyl)-propionic acid, which is one of the two most important members of a class of 2-aryl-propionic acids with non-steroidal anti-inflammatory activity used, for instance, in the management of arthritis. In this case, the S(+) enantiomer of the drug is known to be 28 times more therapeutically potent that its R(−) counterpart. Still another example of chiral pharmaceuticals is provided by the family of beta-blockers, the L-form of propranolol is known to be 100 times more potent that the D-enantiomer.
Synthesis of chiral compounds by standard organic synthetic techniques generally leads to a racemic mixture which, in the aggregate, may have a relatively low specific bioactivity since certain of the stereoisomers in the mixture are likely to be biologically or functionally inactive. As a result, larger quantities of the material must be used to obtain an effective dose, and manufacturing costs are increased due to the co-production of stereochemically “incorrect” and hence, inactive ingredients.
In some instances, certain isomers may actually be deleterious rather than simply inert. For example, the D-enantiomer of thalidomide was a safe and effective sedative when prescribed for the control of morning sickness during pregnancy. However, its L-thalidomide counterpart was discovered to be a potent mutagen.
Methods are available for stereoselective synthesis that generally involve chemical synthesis and isolation steps that are lengthy, complex and costly. Moreover, a synthetic scheme capable of producing one specific enantiomer cannot be applied in a general way to obtain other optically active compounds. What is needed is a generalized approach to the resolution of racemic mixtures produced by ordinary chemical reactions, and a number of approaches have been used.
A widely used approach has been the selective precipitation of desired compounds from racemic mixtures. See, for example, Yoshioka et al. [U.S. Pat. No. 3,879,451], Paven et al. [U.S. Pat. No. 4,257,976], Halmos [U.S. Pat. No. 4,151,198], and Kameswaran [U.S. Pat. No. 4,454,344].
The above procedures successfully resolved racemic mixtures because treatment of the mixtures with optically pure reagents produced diastereomers which, unlike the initial racemic compounds, have different physical properties. Thus, fractional crystallization or other physical means may be employed to separate diastereomeric compounds.
Separation of diastereomers can also be carried out by chromatography. For example, Pollock et al. [J. Gas Chromatogr. 3: 174 (1965)] have resolved diastereomeric amino acids by gas chromatography. Mikes et al. [J. Chromatogr. 112:205 (1976)] have used liquid chromatography to resolve diastereomeric dipeptides. In most cases, the optically pure reagents have been in the stationary phase during chromatographic separation, but they may also be used in elutants. Hare et al. [U.S. Pat. No. 4,290,893] have used liquid chromatography to resolve racemic mixtures that were treated with aqueous elutants containing optically pure reagents and metal cations; resolution occurred because the resulting diastereomeric complexes had different partition coefficients in the chromatographic system.
All of the methods described to this point have relied upon the availability of suitable optically pure reagents, but such reagents are often not available or else their use is prohibitively expensive. In an alternative approach, enzymatic resolution techniques have been developed. Many different classes of enzymes have been used for the resolution of stereoisomers on a preparative scale, including hydrolases (especially the lipases and esterases such as chymotrypsin), lyases, and oxidoreductases (e.g., amino acid oxidases and alcohol reductases). Generally speaking, enzymes for use in resolutions should ideally exhibit broad substrate specificity, so that they will be capable of catalyzing reactions of a wide range of “unnatural” substrates, and a high degree of stereoselectivity for catalyzing the reaction of one isomer to the exclusion of others.
The hydrolases (e.g., lipases and esterases) are among the more attractive enzymes for use in resolutions, because they do not require expensive cofactors, and some of them exhibit reasonable tolerance to organic solvents. Additionally, chiral chemistry often involves alcohols, carboxylic acids, esters, amides, and amines with chiral-carbons, and carboxyl hydrolases are preferred choices as stereoselective catalysts for reactions of such species. For instance, enzymatic treatment has been applied to the resolution of racemic mixtures of amino acid esters. Stauffer [U.S. Pat. No. 3,963,573] and Bauer [U.S. Pat. No. 4,262,092].
Separation of reaction products from enzymes has been facilitated by attaching the enzyme to a solid support which could be removed by centrifugation or packed into a column through which the racemic mixtures were passed.
Enzymes have also been explored for the resolution of classes of compounds other than the amino acids discussed above. Immobilized lipase in principal resolves mixtures by enzymatic hydrolysis or transesterification. In the case of a biphasic hydrolysis reaction, the differing solubility properties of the acids and esters involved required the dispersion and agitation of mixtures containing the immobilized solid-phase enzyme, an aqueous buffer, and the water-immiscible organic phase containing solvent and reactant—a relatively inefficient process.
Enzymes have been applied to the resolution of optical isomers of insecticides. For instance, Mitsuda et al. [Eur. Patent Appl'n. Publ. No. 0 080 827 A2] contacted a racemic acetic acid ester with stereoselective esterases of microbial and animal origin in biphasic systems (i.e., aqueous/organic dispersion). In related work on optically purified pyrethroids, Mitsuda et al. [U.S. Pat. No. 4,607,013] employed microbial esterases. Klibanov et al. [U.S. Pat. No. 4,601,987] resolved racemic 2-halopropionic acids by means of lipase-catalyzed esterification reactions conducted in organic media.
Additional examples can also be provided of the state-of-the-art enzyme-mediated resolution as applied to the production of optically purified pharmaceuticals. Sih [U.S. Pat. No. 4,584,270] has disclosed enzymatic means for the production of optically pure (R)-4-amino-3-hydroxybutyric acid, a key intermediate in the preparation of L-carnitine.
Until recently only naturally occurring L-enzymes could be described, and this left the presumed properties of D-enzymes, including their folded structures, enzymatic activity and chiral specificity, as unexplored questions. What was needed was sufficient progress in the chemical synthesis of proteins to make possible the total synthesis of the D-enantiomer of whole enzymes in sufficient quantity to form crystals and to perform other functions.